As far as we can tell, it’s mostly electrons, neutrons, and protons getting flung out into these jets. In the end, it turns out that the electrons are the key factor in making the jets so visible to our telescopes.
The jets themselves are interesting objects. Not every galaxy’s black hole produces a jet, even though all sizable galaxies have black holes. It seems that in order to produce this jet, the supermassive black hole in the very center of the galaxy has to be actively trying to gather new material into itself. Supermassive black holes are hilariously inefficient at growing larger, even when there’s material around for it to work with. Our Milky Way’s supermassive black hole isn’t growing at the moment because there’s no material nearby, but even if there were a lot more gas and dust very near the black hole, the black hole wouldn’t be able to grow very fast.
Part of the reason a black hole doesn’t grow very fast is that the material orbiting a black hole has to continue to lose energy to keep falling into the black hole, and that process of energy loss is driven by inefficient things like friction and heat. On top of all of this, there are probably crazy things happening with the magnetic fields within the rapidly rotating material around the black hole. Magnetic fields are a bit of a bugbear for studies of galaxies – we know that there are magnetic fields around, but we’re not quite sure how much of an effect they have on the galaxy, and they’re stupendously difficult to model correctly.
In the case of jets, we know that there must be strong magnetic fields, because we observe a type of glow that only happens if you have both very rapidly moving electrons and a magnetic field. It’s called synchrotron radiation, and its happens when you get a relativistic electron (which means that it’s moving at a significant fraction of the speed of light) caught in a twisting orbit around a magnetic field line. The electron moves in a helix around the magnetic field line, and emits light that we can observe with a wide range of different telescopes.
It’s usually this synchrotron radiation that we see in the images of jets coming from a supermassive black hole. If you’re looking at a radio image or an optical image, what you’re looking at is the glowing byproducts of nearly speed-of-light electrons bending under the influence of a magnetic field.
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